As one of the methods for synthesizing liquid fuels from natural gas, the GTL (Gas To Liquids: liquid fuels synthesis) technique is known. The GTL technique is a technique of producing liquid fuel products, such as naphtha (raw gasoline), kerosene, gas oil, and wax, through the steps of reforming a natural gas to produce a synthesis gas containing a carbon monoxide gas (CO) and a hydrogen gas (H2) as main components, synthesizing hydrocarbon compounds (hereinafter also refereed to as “FT synthesis hydrocarbons”) using this synthesis gas as a feedstock of the Fischer-Tropsch synthesis reaction (hereinafter also referred to as “FT synthesis reaction”), and hydrogenating and fractionally distilling the FT synthesis hydrocarbons. The liquid fuel products using the FT synthesis hydrocarbons as a feedstock have high paraffin content, and do not include sulfur components, for example, as shown in Patent Document 1, the liquid fuel products have attracted attention as environment-friendly fuels.
As techniques of reforming natural gas, for example, reforming methods using carbon dioxide gas, such as steam and carbon dioxide gas reforming method and a carbon dioxide gas reforming method, are known. In case of reforming a natural gas using the carbon dioxide gas, an unreacted carbon dioxide gas is contained in the synthesis gas in a relatively high concentration. Therefore, a step of separating the carbon dioxide gas from the synthesis gas before the FT synthesis reaction may be adopted. The separated carbon dioxide gas is reused for reforming the natural gas.
In the FT synthesis reaction, heavy FT synthesis hydrocarbons with a relatively large carbon number are produced as a liquid, and various kinds of gases are generated as gaseous by-products. In the gaseous by-products, for example, a carbon dioxide gas, steam, hydrocarbon compounds with a carbon number of two or less, and hydrocarbon compounds with a carbon number of three or more are included. The gaseous by-products are discharged along with unreacted synthesis gas from a synthesis reactor which performs FT synthesis reaction. The discharged gaseous by-products are introduced into a separator and condensed light liquid hydrocarbon compounds are separated from the gaseous by-products. The remaining gas component is reused as necessary. For example, the unreacted synthesis gas is reused for the FT synthesis reaction, and the carbon dioxide gas is used for reforming the natural gas.
The separation of the carbon dioxide gas from the synthesis gas or gaseous by-products can be performed, for example, by using an absorbent which absorbs the carbon dioxide gas at room temperature, and releases the carbon dioxide gas when it is heated. For example, absorbing carbon dioxide gas included in the synthesis gas and the gaseous by-products by passing those gases through the absorbent at room temperature, and stripping the carbon dioxide gas from the absorbent by heating the absorbent. The absorbent which has released the carbon dioxide gas is reused for separating the carbon dioxide gas from the synthesis gas and the gaseous by-products. Conventionally, in a step of separating the carbon dioxide gas from the synthesis gas and in a step of separating the carbon dioxide gas from the gaseous by-products, circulation systems of the above absorbent are individually provided in the respective steps.